How to Start Seeds Indoors: The Number Is Two Inches

Skip the windowsill

This is where nearly every seed-starting article puts the tray, usually in the photograph at the top. University of Minnesota Extension heads a whole section "Avoid the windowsill" and gives two reasons.

The first is temperature. "Window sills can be the coldest place in the house, especially at night, and then the hottest during the day." A seedling that swings from cold to hot every twenty-four hours is being asked to grow in a place with worse conditions than the rest of the room.

The second is the light itself, which sounds wrong until you read it twice. "Sunlight in Minnesota gains strength through April and May, but it is relatively weak compared to artificial light sources kept close to the plants." And earlier on the same page: "It's much better to grow seedlings under fluorescent or LED lights than to rely solely on natural light, even in a greenhouse."

Even in a greenhouse. A window gives a plant a slice of sky through glass at whatever angle the season allows. A cheap lamp two inches away gives it far more, all day, at the same intensity in March as in May. That is the whole reason the number below matters.

The consequence Minnesota gives is the one you will recognize: "Windowsill-grown seedlings tend to be too tall, with thin, bent stems."

Two to four inches, twelve to sixteen hours

Here is the instruction the rest of the internet rounds off into "adequate light."

University of Minnesota Extension: "Keep lights no more than 4 inches above the tops of your seedlings: as close as 2 inches is ideal."

Two to four inches. Not a foot, not bright, not near a window. And because the seedlings grow every day, the lamp has to move with them — Minnesota's own suggestion is practical: "Hang lights from chains to ease raising them as the plants grow."

Then the duration, which has a limit at both ends. "Plants need 12 to 16 hours of light daily. Don't leave the lights on continuously, as many plants need some dark period each night to develop properly." A timer does this and costs less than a tray of seedlings.

And you do not need a special lamp. Minnesota: "Standard fixtures with two "cool white" fluorescent tubes per fixture also give plants adequate light and are inexpensive." The distance is doing the work here, not the spectrum.

Leggy seedlings are a light problem, and it has a name

If you have grown seedlings before, you have seen this: tall, pale, thin-stemmed things leaning hard in one direction, falling over under their own weight.

Minnesota names the cause without hedging: "Lack of light is the major cause of elongated, skinny stems."

So it is not a watering problem, a feeding problem or bad seed. A stretching seedling is a plant spending everything it has on reaching for a light source that is too far away — which is exactly what a windowsill is, and exactly what a lamp a foot above the tray is.

The honest part: this is a prevention answer rather than a cure. A stem that has stretched does not un-stretch. Moving the light closer stops it getting worse and the growth that comes after will be stockier, but the drawn-out section stays. Our timing page notes one workaround, and it is one crop: a leggy tomato can be planted deep and will root along the buried stem. Most things cannot.

The mix, and why not garden soil

The other half of a successful tray is what the seed sits in, and Minnesota is specific about it.

1. Use a commercial seed-starting mix. Minnesota describes them as usually composed of peat and vermiculite, without any true soil — sterile, lightweight and free from weed seeds, with a texture suited to germinating seeds and tiny seedlings. Garden soil brings weed seed and disease into a tray of things too small to survive either.
2. Fill the containers and water the mix before you sow. Minnesota's sequence, and the reason is in the next line of it: the potting mix will settle down into the containers. Watering first means you find out how far it settles before the seed is in it rather than after.
3. Sow to the packet, and sow two if the seed is old. Minnesota says to sow fresh seed individually per container according to package directions, and to plant two or more seeds per cell when using older seed with lower germination rates.

Warmth to germinate, then ordinary room conditions to grow. Minnesota: "Air temperatures should be kept above 60°F and adequately warm if bottom heat is provided." Bottom heat helps things germinate; it is not something the seedlings need for the rest of their time indoors.

If seedlings collapse at soil level a few days after emerging, that is a different problem with its own answer — damping off.

Thin with scissors, not fingers

A small instruction that saves the plant you are keeping.

Minnesota: "Once the seedlings have developed true leaves, cut all but the healthiest one off at ground level with scissors. Avoid separating or pulling the unwanted seedlings, as this could damage the roots of the ones you want to keep."

Two cells' worth of roots are tangled together by the time you can tell which seedling is stronger. Pulling one out takes some of the other with it. Cutting the loser at the surface leaves its roots where they are to rot down, and the winner never notices.

"True leaves" is the timing marker and it is worth being sure of: the first pair a seedling opens are seed leaves, usually rounded and plain. The next pair look like the plant is supposed to look. That second pair is the cue.

And the timing is a different question

Everything above is method. When to actually start is a separate decision and it has its own page, because it hangs on a date rather than on equipment.

The short version: you count backwards from your last spring frost, and the number of weeks differs by crop. The timing page has the crop-by-crop table, along with the crops that should not be started indoors at all and what to do if you have already started too early.

And at the far end of the indoor stretch there is one more step that is not optional: hardening off. A seedling raised two inches under a lamp has never met wind.

Seed-starting questions people actually ask

Sources

Every planting date, depth, and temperature on this page traces to these university extension guides. Where they differ, the ranges above hold the whole spread.

1. University of Minnesota Extension — Starting seeds indoors